Dielectric films for narrow gap-fill applications

ABSTRACT

A colloidal suspension of nanoparticles composed of a dense material dispersed in a solvent is used in forming a gap-filling dielectric material with low thermal shrinkage. The dielectric material is particularly useful for pre-metal dielectric and shallow trench isolation applications. According to the methods of forming a dielectric material, the colloidal suspension is deposited on a substrate and dried to form a porous intermediate layer. The intermediate layer is modified by infiltration with a liquid phase matrix material, such as a spin-on polymer, followed by curing, by infiltration with a gas phase matrix material, followed by curing, or by curing alone, to provide a gap-filling, thermally stable, etch resistant dielectric material.

FIELD OF THE INVENTION

The present invention relates generally to dielectric materials for usein semiconductor devices, and, more specifically to dielectricmaterials, prepared from colloidal dispersions, that have high thermalstability and etch resistance and completely fill narrow gaps.

BACKGROUND

In order to provide integrated circuits (ICs) with increasedperformance, the characteristic dimensions of devices and spacings onthe ICs continue to be decreased. Fabrication of such devices oftenrequires the deposition of dielectric materials into features patternedinto layers of material on silicon substrates. In most cases it isimportant that the dielectric material completely fill such features,which may be as small as 0.01 to 0.05 μm or even smaller in nextgeneration devices. Filling such narrow features, so-called gap filling,places stringent requirements on materials used, for example, forpre-metal dielectric (PMD) or shallow trench isolation (STI)applications. The pre-metal dielectric layer on an integrated circuitisolates structures electrically from metal interconnect layers andisolates them electrically from contaminant mobile ions that degradeelectrical performance. PMD layers may require filling narrow gapshaving aspect ratios, that is the ratio of depth to width, of five orgreater. After deposition, the dielectric materials need to be able towithstand processing steps, such as high temperature anneal, etch, andcleaning steps.

Dielectric materials are commonly deposited by chemical vapor deposition(CVD) or by spin-on processes. Each of these approaches has somelimitations for filling very narrow gaps. Plasma enhanced chemical vapordeposition (PECVD) processes provide high deposition rates atcomparatively low temperatures (about 400° C.). The main drawback isthat PECVD processes have a lower deposition rate inside a gap than atother locations on a surface. The differential deposition rates cancreate structures overhanging a gap opening, leading to voids within thegap. Typically, for spacings less than 0.25 μm, depending on the aspectratio, it is difficult to achieve void-free gap fill using standardPECVD approaches.

Phosphosilicate glass (PSG) and borophosphosilicate glass (BPSG) arecommonly used for premetal dielectric applications. The films areusually deposited using atmospheric pressure CVD (APCVD),sub-atmospheric pressure CVD (SACVD) or low pressure CVD (LPCVD).Depending on the process conditions and precursors used these methodscan achieve an almost conformal coating. Gap-fill is achieved by apost-deposition reflow process in which the material is treated at hightemperatures, typically 800-1200° C. The inclusion of phosphorous andboron, in particular, the boron, in the glass lower the glass transitionand flow temperatures. However, the use of CVD followed by reflow infuture advanced devices will be limited by the high thermal budgetrequired for the reflow process, which is not compatible with certainmaterials and processes, such as cobalt silicide used at the contactlevel. For very narrow gaps, less than 0.2 μm, there is an increasingrisk that voids may remain, even after high temperature processing.

Some workers have used high density plasma chemical vapor deposition(HDP CVD) to improve gap-fill of PSG and BPSG. In the high densityplasma process, deposition and etching occur simultaneously. Etching ismost efficient at the top comers of narrow openings, therebycompensating for a lower deposition rate inside the gap. HDP CVDdeposition does not require high temperature processing, although ananneal step can be used if a denser film is desired. The HDP CVD processhas the drawback that for narrower structures, lower deposition to etchratios have to be used resulting in a relatively slow overall fillingrate. Improved gap-fill may also require modifications in the design ofdevice features, such as rounded comers and sloped sidewalls. Finally,there is also a concern about plasma damage to the device during HDP CVDprocessing.

Spin-on glasses and spin-on polymers such as silicates, siloxanes,silazanes or silisequioxanes generally have good gap-fill properties.The films of these materials are typically formed by applying a coatingsolution containing the polymer followed by a bake and thermal cureprocess. The utility of these spin-on materials may be limited, however,by material shrinkage during thermal processing. Thermal shrinkage is akey consideration for materials which have to withstand high processtemperatures, such as materials used for pre-metal dielectric and/orshallow trench isolation applications, which may involve processtemperatures exceeding 800° C. High shrinkage can lead to unacceptablefilm cracking and/or formation of a porous material, particularly insidenarrow gaps. Cracked or porous material may have an undesirably high wetetch rate in subsequent process steps.

Thus there remains a need for a dielectric material that providesvoid-free gap-fill of narrow features at processing temperatures lessthan the reflow temperatures used currently. The gap-filling materialsneed to have high thermal stability and reasonable resistance to etchingsolutions to survive subsequent processing steps.

SUMMARY

A colloidal dispersion of particles composed of a dense materialdispersed in a solvent is used in forming a gap-filling dielectricmaterial with low thermal shrinkage. The particles are preferably ofnanometer-scale dimensions and are termed nanoparticles. The densematerial is either a dielectric material or a material convertible to adielectric material by oxidation or nitridation. The dielectric materialis particularly useful for pre-metal dielectric and shallow trenchisolation applications. Oxides and nitrides of silicon, oxides andnitrides of aluminum, and oxides and nitrides of boron are useful asnanoparticle materials. Colloidal silica is particularly useful as thecolloidal dispersion. The dielectric material optionally includes dopantspecies such as arsenic, antimony, phosphorous, or boron.

According to the methods of forming a dielectric material, the colloidaldispersion is deposited on a substrate and the deposited film is driedforming a porous intermediate layer. The intermediate layer is modifiedby infiltration with a liquid phase matrix material, followed by curing,where, in all cases, curing includes optionally annealing, byinfiltration with a gas phase matrix material, followed by curing, or bycuring alone, to provide a gap-filling, thermally stable, etch resistantdielectric material.

Infiltrating matrix materials applied in the liquid phase are spin-onpolymers, including oligomers and monomers, that can be converted tosilica or similar ceramic materials on high temperature cure, optionallyin the presence of oxygen or steam. The matrix materials include, butare not limited to, silicates, hydrogen silsesquioxanes,organosilsesquioxanes, organosiloxanes, silsesquioxane-silicatecopolymers, silazane-based materials, polycarbosilanes, andacetoxysilanes. The liquid matrix materials optionally include dopantspecies such as arsenic, antimony, phosphorous, or boron. In liquidphase infiltration, a coating solution of the matrix material is appliedon the colloidal film.

Gas phase infiltration uses chemical vapor deposition (CVD) methodsunder conditions in which impinging molecules have a low stickingcoefficient and/or high surface diffusion to avoid sealing a top surfaceof a narrow gap before bulk porosity of the intermediate layer isreduced. The CVD deposited materials optionally include dopant species.Other gas phase deposition processes, such as atomic layer deposition,may also be used for gas phase infiltration.

The dried intermediate layer or the infiltrated intermediate layer iscured, for example in a furnace at temperatures of between about 600 and800° C. or by rapid thermal processing, for example, at temperatures ofbetween about 700 and 900° C. Optionally, one or more bake steps attemperatures, for example, between about 75 and 300° C. precede thecuring process. In addition, the cure may be followed by a highertemperature annealing step. Curing, and optionally annealing, inducessintering of the nanoparticles and reflow of the infiltrated matrixmaterial. Inclusion of dopant species in the nanoparticles or in thematrix materials lowers the reflow temperature.

Films prepared according to the present invention fill narrow gaps, lessthan 100 nm and as small as 50-60 nm in width, without voids or cracks.They do not show delamination of cracking even after heat treatment at900° C. The cured films have minimized open porosity as evidenced bytheir resistance to etchant solutions in filled gaps. Further, theaverage etch rate of cured films on blanket wafers is on the same orderas the average etch rate of silicon dioxide films produced by chemicalvapor deposition. Thus, films prepared according to the presentprocesses are advantageously used as pre-metal dielectric and shallowtrench isolation materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 are flow diagrams of processes of forming a dielectricmaterial according to embodiments of the present invention.

FIG. 4 is a pre-metal layer filled with a dielectric material formedaccording to an embodiment of the present invention.

FIG. 5 is a shallow trench isolation structure filled with a dielectricmaterial formed according to embodiments of the present invention.

DETAILED DESCRIPTION

Methods of forming a gap-filling dielectric material with low thermalshrinkage make use of a colloidal dispersion of dense particles. Acoating solution of the colloidal dispersion is deposited on a substrateto form a film, and the film is intentionally modified by one or moreapproaches. The dielectric material is beneficially used for pre-metaldielectric and shallow trench isolation applications.

Key to the present invention is the nature of the colloidal dispersion.Colloids are generally defined as systems in suspension in which thereare two or more phases. In the colloidal suspension, one of the phases,termed the dispersed phase, is distributed in the: other phase, termedthe continuous phase. A familiar type of colloid consists of dispersionsof small particles in a liquid.

According to one aspect of the present invention, a colloidal dispersionof nanometer scale particles, termed nanoparticles, composed of a densematerial dispersed in a solvent is used. The dense material is either adielectric material or a material convertible to a dielectric materialby reaction with oxygen or nitrogen. In addition, the physical size ofthe nanoparticles needs to be substantially unchanged by thermalprocessing. The size of the nanoparticles should not be reduced by morethan 10% when exposed to temperatures of about 700° C. In the event thenanoparticles are composed of a material such as silicon or aluminum,convertible to a dielectric material by oxidation or nitridation, theparticle size may increase somewhat during curing in the presence of anoxygen- or nitrogen-bearing species.

Silicon and aluminum and refractory oxides and nitrides of silicon andaluminum are useful nanoparticle materials. Additional useful materialsinclude nitrides, such as boron nitride and gallium nitride, and alsoboron oxide and boron carbide. Suitable dense, silicon-containingmaterials for use as nanoparticles include silica, silicon, siliconnitride, silicon oxynitride, and combinations and mixtures thereof. Forexample, colloidal silica is advantageously used as the colloidaldispersion. Methods for forming colloidal silica are known in the art asdescribed, for example, in U.S. Pat. No. 3,634,558 and in Van Helden etal., (J. Colloid Interface Sci. 81, 354 (1981)), both of which areincorporated herein by reference. In addition, colloidal silica isavailable commercially. A general criterion is that the nanoparticlesexperience little or no chemical change during a high temperature cureprocess with the exception of oxidation or nitridation, as discussedabove. However, as long as the low shrinkage criterion is met,nanoparticles may additionally contain small quantities of siliconpolymers such as hydridosilsesquioxanes, organosiloxanes,organosilsesquioxanes, and perhydrosilazanes.

The nanoparticles have a characteristic dimension between about 2 nm andabout 50 nm. The colloidal dispersion has excellent gap-fillingcapability, limited only by the particle size. For any specificapplication, the particle size is chosen to be smaller than the width ofthe opening to be filled. The size distribution of the nanoparticles maybe monodisperse, bimodal, or polydisperse. Bimodal distributions may betailored to provide a higher packing density of nanoparticles, in whichsmaller particles fit into voids generated by packing of largerparticles. The nanoparticles are dispersed in an organic solvent orinorganic solvent, such as an aqueous solvent or solvent mixture, or ina supercritical fluid. Suitable organic solvents include solventscommonly used in coating solutions of spin-on polymers, such asmethanol, ethanol, isopropyl alcohol, methylisobutylketone,cyclohexanone, acetone, and anisole, to name only a very few. The solidcontent of nanoparticles in the colloidal dispersion typically rangesfrom as little as 0.5 weight % to as much as 20%. Higher or lowerconcentrations may be used to adjust the coating thickness. Additionaladditives such as surfactants or binders may also be present in thedispersion. Infiltration matrix materials described below are useful asbinders when added to the dispersion in small quantities, for example ina ratio of nanoparticle to binder greater than about 10:1.

According to another aspect of the present invention, the nanoparticlesinclude dopant species such as arsenic, antimony, phosphorous, or boron.Including such dopants may enhance material properties of the filmformed from the colloidal dispersion. For example, dopants areintroduced to increase mobile ion gettering and to lower the glasstransition temperature. Boron is used, among other reasons, to produce adielectric material with increased etch resistance.

A film of the colloidal dispersion is typically formed on a substrate byspin coating. Other methods known in the art for applying coatingsolutions such as dip coating or spray coating may alternatively beused. The coated film is dried evaporating the solvent in thedispersion. The coated film may be dried during the spin-coatingprocess, for example by a fast spin, or by a rest period following thedispense step. Alternatively, the coated film may be heat treated bymethods such as lamp heating, baking on a hot plate, at one or moretemperatures between about 75 and 300° C., or by other methods known inthe art. In addition to evaporating the solvent, the heat treatment mayserve to keep the particles attached to the substrate, so that they arenot removed/rinsed away during the subsequent infiltration. The heattreatment may also have the effect of reacting the optional bindermaterial. The film thus formed on the substrate is typically porous innature with an open pore structure. For applications such as PMD or STIlayers, it is desirable to minimize porosity of the film in order tominimize thermal shrinkage and moisture sensitivity and to maximizethermal stability and etch resistance. It is also desirable to changethe open pore structure to a closed pore structure. Eliminating openporosity improves the chemical resistance such as etch resistance of thematerial during subsequent processing steps. To minimize open porosity,a film formed from the colloidal suspension is modified by one or moreapproaches. The processes include infiltration with a matrix materialapplied in the liquid phase, infiltration with a matrix material appliedin the gas phase, and cure/anneal processes.

Infiltrating matrix materials applied in the liquid phase are spin-onpolymers that can be converted to silica or similar ceramic materialsduring a cure process. An example is a high temperature cure, optionallyin the presence of oxygen or steam. The term spin-on polymer, as usedhere, includes oligomers and monomers, as well as polymers. The matrixmaterials include, but are not limited to, silicates, hydrogensilsesquioxanes, organosilsesquioxanes, organosiloxanes,organhydridosiloxanes, silsesquioxane-silicate copolymers,silazane-based materials, polycarbosilanes, and acetoxysilanes. Suitablecommercial matrix materials include silicates of the T11 and T14 series,the spin-on-glass Accuspin™, and the organohydridosiloxane HOSP™, allprovided by Honeywell International, Inc. (Morristown, N.J.).

In liquid phase infiltration, a coating solution of the matrix materialis applied on the colloidal film, typically by spin coating, althoughalternative application methods may be used. Low molecular weight matrixmaterials have the benefit of more easily penetrating narrow spacesbetween the nanoparticles. Matrix materials with molecular weightsranging from several hundreds to multiple thousands of atomic mass units(amu) may be desirable. Infiltrating molecules with hydrodynamicdiameters less than 1-2 nm are advantageous to infiltrate small pores.Solid content of the infiltrate solution may range from nearly 100%solid to as low as 2% solid. The infiltrating matrix material occupiesspace between the nanoparticles, minimizing open porosity. In addition,depending on the molecular weight of the matrix polymer, the matrixmaterial may also form an overlayer on top of the infiltrated colloidallayer, as described, for example, in Example 5 below.

In some embodiments, dopant species, such as arsenic, antimony,phosphorous, or boron, as described above, are included in the matrixmaterials. Dopants are introduced, for example, to increase mobile iongettering and to lower the glass transition temperature of thegap-filling dielectric material. The introduction of phosphorous hasbeen observed to provide better gap-filling properties and boron isused, among other reasons, to produce a dielectric material withincreased etch resistance and to provide coating solutions for applyingthe dielectric material with longer shelf life. Dopants may be providedfor other reasons, as well. In particular, boron and phosphorous dopedsilicates and silsesquioxanes are useful doped matrix materials.Suitable phosphorous doped silicates include the phosphosilicateproducts P062A, P082A, P112A, P064A, P084A, and P114A, all provided byHoneywell International, Inc. (Morristown, N.J.). A boron dopedsilsesquioxane is prepared, for example, by adding a solution of boronoxide in isopropyl alcohol to a silsesquioxane solution as describedbelow in Example 3. A boron and/or phosphorous doped silicate isprepared by including a boron and/or phosphorous precursor in thesynthesis of the silicate polymer. Suitable phosphorous precursorsinclude but are not limited to P₂O₅, H₃PO₄, and trialkylphosphates, suchas trimethylphosphate, and triethylphosphate. Suitable boron precursorsinclude but are not limited to B₂O₃, H₃BO₃, and trialkylborates, such astrimethylborate, and triethylborate. The synthesis of boron andphosphorous doped silicate using P₂O₅ and B₂O₃ as reactants is describedbelow in Example 7. Alternatively, the matrix material may comprise amixture of the undoped spin-on polymer and the boron and/or phosphorousprecursors described above. Additional dopant-containing moleculesuseful in dopant/matrix material mixtures include phosphazenes,borazenes, and borophosphates.

In an alternative embodiment, the matrix material is included as acomponent of the colloidal dispersion. Pre-mixing the colloidaldispersion and the matrix material has the advantage of eliminating theprocess step of applying a coating solution of the matrix material. Inthe case of pre-mixing, the fraction of added matrix material is suchthat the majority of the volume in the gap is occupied by thenanoparticles in order to minimize thermal shrinkage of the dielectricmaterial. For example, the ratio of nanoparticles to matrix material isgreater than or about 1:1. Note that when the matrix material is used asa binder in the colloidal dispersion, the ratio of nanoparticles tomatrix material is much greater than when the matrix material ispremixed in the colloidal suspension.

Gas phase infiltration with matrix materials provides a second approachto densification. Gas phase infiltration processes include chemicalvapor deposition (CVD) and atomic layer deposition. For use as aninfiltration process, CVD conditions are adjusted to avoid sealing a topsurface before bulk porosity is reduced. The CVD process is performed,therefore, in a reaction rate limited process regime, achieved, forexample, by conditions in which impinging molecules have a low stickingcoefficient and/or high surface diffusion. The sticking coefficient isthe probability of an arriving molecule to react on the surface. Asticking coefficient of 1 means that every molecule will remain on thesurface. For successful infiltration using CVD, the sticking coefficienthas to be smaller than 0.01. For a thermal CVD process this can usuallybe achieved by choosing a deposition temperature lower than the standardprocess temperature. For typical CVD deposition of material layers onflat surfaces, such conditions would result in unacceptably lowdeposition rates. However, in the present application to deposition on ahighly porous film, the surface area can be many times larger than theflat surface substrate area, enhancing the effective deposition rate topractical levels. An exemplary gas phase densification process is CVDdeposition of tetraethoxysilane (TEOS) and oxygen at a temperaturebetween about 400 and 600° C. Infiltration deposition temperatures aresignificantly lower than the about 600 to 700° C. process conventionallyused for CVD of the TEOS/oxygen system. Dopants, as listed above, can beincluded in the CVD infiltration process by including dopant precursorgases along with the CVD reactant gases. Examples of dopant precursorsmay include boron oxides, triethylborate, alkylboranes, and diborane forboron doping, and triethylphosphate, trimethylphosphate, and phosphinefor phosphorous doping.

Atomic layer deposition (ALD) provides an alternative gas phaseinfiltration process. In ALD, each atomic layer is deposited byalternatively supplying a reaction gas and a purging gas. Thus, ALD isan excellent approach to forming a conformal, uniform coating of theentire pore surface structure of the colloidal dispersion layer. Anexample for an ALCVD process for a depositing Al₂O₃ uses Al(CH₃)₃ andwater vapor as precursors. An ALCVD coating consisting of less than20-30 atomic layers is beneficially used in the present invention.Larger numbers of atomic layers may also be used.

Curing processes, which may include thermal processing or annealingmethods, such as electron beam annealing or ion beam annealing, orcombinations thereof, provide a third approach to modification of theopen pore structure. The curing process may be applied to theas-deposited colloidal dispersion, or to the film after a liquid phaseor gas phase infiltration process has been performed. All methods offorming a gap-filling dielectric material according to the presentinvention include a curing step. Optionally, one or more bake steps attemperatures between about 75 and 300° C. may precede the curingprocess. The film is cured in vacuum or in an atmosphere of commonlyused gases such as nitrogen, oxygen, ozone, steam, ammonia, argon,carbon monoxide, carbon dioxide, nitrous oxide, nitric oxide, helium,hydrogen, forming gas, or mixtures thereof. For nanoparticles of amaterial, such as silicon, convertible to a dielectric by oxidation ornitridation, a curing atmosphere containing an oxygen or nitrogenbearing species is used.

An exemplary curing process is curing in a furnace at temperaturesbetween about 600 and 800° C. in a nitrogen/oxygen atmosphere for a timeperiod less than about 2 hours. Alternatively, the film is cured byrapid thermal processing (RTP) at a temperature between about 700 and900° C. for a period of about 10 seconds to 5 minutes. Optionally,particularly for processes in which neither the nanoparticles nor thematrix material are doped materials, curing may be followed by a highertemperature thermal annealing step. An exemplary annealing process usestemperatures between about 800 and 1000° C. in a nitrogen atmosphere. Inthe present processes, curing modifies the dielectric film everywhere ona substrate including inside narrow gaps and in open regions.

The present processes are used to deposit dielectric materials onsubstrates in the fabrication of integrated circuit devices. Integratedcircuit devices include but are not limited to silicon based devices,gallium arsenide based devices, opto-electronic devices, focal planearrays, photovoltaic cells, and optical devices. As is well known,integrated circuit devices generally include a substrate, conductivecircuit lines, and dielectric material. In addition, barrier layers,etchstop layers, and conducting gates, such as polysilicon gates, areincluded in the devices. The interconnected circuit lines function todistribute electrical signals in the device and to provide power inputto and signal output from the device. Integrated circuit devices willgenerally include multiple layers of circuit lines which areinterconnected by vertical metallic studs, that is metal-filled vias.Suitable substrates include, but are not limited to, silicon, silicondioxide, glass, silicon nitride, ceramics, and gallium arsenide. As usedherein, substrates refers to any of the layers, planarized or havingtopography, including, semiconducting wafers, dielectric layers, gates,barrier layers, etchstop layers, and metal lines found in integratedcircuit devices.

The principal methods of forming a gap-filling dielectric materialaccording to the present invention are summarized in the process flowdiagrams of FIGS. 1-3. FIG. 1 describes a process 10 of forming a filmwithout infiltration with a matrix material. A colloidal dispersion ofnanoparticles is deposited on a substrate, at step 12, followed by anoptional bake step 14. The nanoparticles may be doped or undoped. Thefinal step of process 10 is a cure process 16 as described above.Process step 16 in FIGS. 1-3 implicitly includes an optional highertemperature anneal. FIG. 1 also describes the alternative process offorming a dielectric material in which matrix material is premixed intothe colloidal dispersion at step 12. When depositing a premixeddispersion, bake step 14 is included in the process.

FIGS. 2 and 3 describe processes 20 and 30 including infiltration with aliquid matrix material and by a gas phase process, respectively. Inprocesses 20 and 30, deposition of the colloidal dispersion, step 12, isfollowed by an optional bake process, 14. For liquid phase infiltration,process 20, at step 26, a coating solution of the matrix material isapplied over the film formed from the colloidal dispersion. The coatingsolution at step 26 may contain dopants including but not limited toboron and/or phosphorous. Step 26 may optionally include multipleapplications of a coating solution of matrix material. Coating thematrix material is followed by a bake step, 28, and cure step 16. In thegas phase infiltration process, 30, after depositing the colloidaldispersion at step 12 and an optional bake process at step 14, a matrixmaterial is deposited on the nanoparticles by chemical vapor deposition,at step 36. Materials containing dopants such as boron and phosphorousare beneficially used at step 36. A cure step 16 completes process 30.Processes 20 and 30 provide a composite gap-filling dielectric materialconsisting of the matrix material surrounding nanoparticle fillermaterial.

The present material is particularly useful for filling narrow gaps witha dielectric material in a pre-metal layer and for filling trenches inshallow trench isolation structures, as illustrated schematically inFIGS. 4 and 5, respectively. Pre-metal layer 40 in FIG. 4 includespoly-silicon gates 43, and a barrier layer 44, on a substrate 45. Gap 42can be even narrower than the “minimum feature size,” as commonlydefined by the limitations of photolithographic methods. The particularembodiment illustrated in FIG. 4 shows dielectric layer 46, includingnanoparticles 47, topped by an overlayer 48 of cured infiltrating matrixmaterial. The matrix material also occupies space between thenanoparticles (not depicted). Using a colloidal silica to provide thenanoparticles and boron-doped silsesquioxane as the infiltrating matrixmaterial, gaps smaller than 100 nm, as small as 50-60 nm in width(0.05-0.06 μm), have been completely filled without delamination, asdescribed in Example 5 below.

An exemplary shallow trench isolation structure 50 depicted in FIG. 5includes substrate 52, pad oxide layer 53, hard mask 54, liner oxide 55,and trench 56 filled with the present dielectric material. Acharacteristic dimension 57 of the shallow trench is typically 1-2 timesthe “minimum feature size.” Thus, STI applications also require amaterial that fills narrow openings. For STI use, nanoparticles aretypically composed of silica, silicon, or mixtures thereof. Infiltrationis performed in the liquid phase or in the gas phase, preferably with anoxide or with matrix material that can form an oxide, such as SiO₂, orAl₂O₃. For gas phase infiltration, silicon oxide may be deposited usingstandard CVD precursors, which include but are not limited totetraethoxysilane (TEOS) and oxygen or ozone, or silicon may bedeposited using precursors which include dichlorosilane ortrichlorosilane.

The benefits of the present gap-filling dielectric material may beunderstood in terms of the microscopic transformations to the materialtaking place during cure step 16, principally the processes of sinteringand reflow. Sintering is conventionally defined as the process ofheating and compacting a powdered material at a temperature below itsmelting point, or glass transition temperature, in order to weld theparticles together into a single rigid shape.

Reflow occurs at temperatures above the glass transition temperature andinduces physical conformational changes. The reflow mechanism for PSGand BPSG materials is described by R. A. Levy (J. Electrochem. Soc., Vol133, No. 7, pp. 1417 (1986)). Reflow is driven by surface tensionforces. The forces are proportional to σ/R², where σ is the surfacetension and R is the radius of curvature. For conventional BPSG reflowthe radius of curvature is on the order of 100 to 500 Å for conformalcoating of narrow gap, whereas the radius of curvature between theparticles in the porous film is on the order of 5 to 50 Å, depending onparticle size. The surface tension forces inside the porous film cantherefore be several orders of magnitude larger than for conventionalcoating, thus inducing reflow at even higher viscosities. Therefore,reflow can convert an initially open pore structure to a closed porestructure at a lower temperature (or shorter time at same temperature)than would be required for conventional reflow.

For process 10, that does not include infiltration with a matrixmaterial, the deposited colloidal dispersion, after removal of thesolvent, consists essentially of the nanoparticles. In these cases,curing may induce sintering of the nanoparticles. Using nanoparticlescontaining phosphorous or boron dopants lowers the minimum temperatureneeded for sintering. Using doped nanoparticles also lowers the glasstransition temperature such that reflow of doped nanoparticles can occurat the furnace cure and RTP temperatures described above.

For processes 20 and 30, which include infiltration with matrixmaterials, the curing process can induce chemical changes in the matrixmaterial. For example, using the silicon-containing spin-on polymerslisted above for liquid phase infiltration, curing in an atmospherecontaining oxygen converts the matrix material to silica or siliconoxynitride. In addition, when the nanoparticles are composed of amaterial convertible to a dielectric, such as silicon, curing in anoxygen or nitrogen atmosphere induces chemical changes in thenanoparticles, as well. The reflow and sintering properties of thematrix material may be tuned through doping. For example using matrixmaterials containing phosphorous or boron lowers the reflow temperatureso that curing induces both chemical and physical change. In the presentcase, reflow occurs at temperatures lower than those required forconventional BPSG glasses. Without being bound to any theory, theinventors attribute the lower reflow temperature to the fact that thedriving force for reflow is surface energy, which is inverselyproportional to radius of curvature. For the colloidal dispersions ofnanoparticles of the present films, the relevant radius of curvature isAngstroms while it is hundreds of Angstroms for conventional BPSG reflowprocesses. In addition, in the present instance, the required flowdistance is more than an order of magnitude smaller.

Thus it may be understood that thermal processing modifies thegap-filling dielectric material, minimizing open porosity, reducing itsporosity, and, in some cases, increasing its density, by sinteringnanoparticles together and/or by inducing reflow in the nanoparticlesand/or in the matrix material. The higher the process temperature, thegreater the resulting modification to the pore structure. Reducing theopen porosity increases the resistance of the dielectric material tocommonly used buffered oxide etchant (BOE) solution, for example asolution containing ammonium fluoride and/or hydrogen fluoride. Asreported in the examples below, dielectric material formed from acolloidal dispersion of silica nanoparticles by thermal processingalone, process 10, and also formed from silica nanoparticles byinfiltration with boron doped hydrogen silsesquioxane, process 20, wasused to fill gaps in a patterned wafer. Both the infiltrated andnon-infiltrated material in the gaps were resistant to a standard 500:1BOE solution. High process temperatures are associated, however, withmaterial shrinkage which can lead to cracking or delamination ofmaterials in narrow gaps. Use of the dense nanoparticles in the presentmethods minimizes thermal shrinkage. For colloidal silica withoutinfiltration for example (see Example 2 below), after annealing at 900°C., total thickness shrinkage of only 4.5% with respect to the bakedfilm has been observed. Therefore, thermal processing temperatures canbe selected according to the present methods to provide gap-fillingdielectric materials that are both crack free and resistant to etchants.

The features and benefits of the present invention are furtherillustrated but not limited by the following experimental examples.

Analytical Test Methods

In characterizing experimental results, refractive index and filmthickness was measured using a Woollam Variable Angle SpectroscopicEllipsometer Model MMA. Film thickness samples were measured post-bake,post-cure, and post-anneal. Percentage shrinkage was calculated as thechange in film thickness divided by the post-bake thickness. Coatedpatterned wafers after bake were cleaved to reveal feature sizes. Thecross section was gold stained with a thin gold layer. Scanning ElectronMicroscope (SEM) images at magnifications ranging from 40,000 to 100,000were obtained using a JOEL JSM model 6330F SEM apparatus.

EXAMPLE 1

Preparation of Colloidal Dispersion of Silica

A 1.8 wt % colloidal silica dispersion in cyclohexanone was prepared bycombining 50.3 gm of a 10 wt % colloidal silica stock solution incyclohexanone (Catalyst and Chemical Industries Co, Japan) with 225 gmcyclohexanone. Average particle diameter of the colloidal silica was10.5 nm. Analysis of the metal concentration of the solution gave: Ca:4.8 ppb; Cr:<1 ppb; Cu:4.2 ppb; Fe:<5 ppb; Mg: 0.7 ppb; Mn:<1 ppb;Ni:<0.5 ppb; K:<5 ppb; and Na: 16 ppb.

A 3.5 wt % colloidal silica dispersion in cyclohexanone was prepared bymixing 90.6 gm of the 10% colloidal silica stock solution with 170 gmcyclohexanone. The solutions were homogenized by ultrasonic agitationfor 30 minutes and then filtered through a 0.1 micron filter beforebeing used in spin coating onto blanket or patterned wafers.

EXAMPLE 2

Coating of the Colloidal Silica Solution

The 1.8 wt % colloidal silica solution of Example 1 was spincoated ontoan 8 inch silicon blanket wafer, hereafter termed a blanket wafer, andonto a patterned wafer with feature sizes ranging from 50 nm to severalmicrons with step heights of 0.2 to 0.4 μm. The spin conditions includeda dynamic dispense for 3 seconds at 300 rpm with dispense volume of 2ml, followed by final spin at 2000 rpm for 20 seconds with accelerationof 50 rpm/sec. The coated wafers were baked at 80° C., 150° C. and 250°C. for one minute each, cured, and annealed. During the cure, thetemperature was ramped from 450° C. to 700° C. at a rate of 5° C./minand then held at 700° C. for 30 minutes. Cure atmosphere was N₂, at 16liter/min and O₂ at 4 liter/min. The wafers were annealed at 900° C. inflowing nitrogen for 20 minutes.

Film thickness and shrinkage of the silica film on the blanket waferafter each processing step are listed in Table 1. Crack-free clear filmswith no visible defects were obtained when deposited on blanket wafers.Film thickness was measured with a spectroscopic ellipsometer (J. A.Woolam VASE.) Film non-uniformity was less than 2%.

TABLE 1 Film Thickness and Shrinkage of Silica Film Film ThicknessRefractive Index Shrinkage from Bake Post-bake 491 Å 1.24 — Post-cure485 Å 1.23 1.5% Post-anneal 478 Å 1.24 4.5%

A refractive index of 1.24 indicated that the deposited film had aporous silica structure. This refractive index value may be comparedwith a refractive index of 1.36 for a 20% porous silica film and with arefractive index of 1.45 for non-porous silica films. The porous filmsshowed very low shrinkage even after heat-treating to as high as 900° C.

The gap-filling ability of the colloidal silica in narrow gaps of thepattern wafers was determined from SEM pictures of the cleaved wafers.SEM pictures were taken for (i) the post-bake, (ii) post-cure and (iii)post-anneal wafers. All three SEM images showed that the silicananoparticles uniformly fill gaps size ranging from 50 nm to severalmicrons in size. There were no delamination or cracking after cure andafter anneal and the nanoparticles also fill the comers around thebottom of the gaps.

EXAMPLE 3

Preparation of Boron-Doped Silsesquioxane Solution

8.2 gm of triethoxysilane, 0.60 gm of deionized water and 0.75 gm of0.02 N nitric acid were added to 41 gm of acetone in a plastic bottle.The mixture was allowed to remain at room temperature for 18 hours andthen diluted with 50 gm of n-propoxy propanol and 20 gm of denaturedethanol to form a base silsesquioxane solution.

3 gm of boron oxide (B₂O₃) was dissolved in 100 gm of isopropyl alcoholto form a 3% boron oxide solution. 6.66 gm of the 3% boron oxidesolution was added to 100.55 gm of the base silsesquioxane solution. Themolecular weight of the boron-doped silsesquioxane resin was 1256 atomicmass units from gel permeation chromatography. The resin contained 2.0%boron by weight. The molecular weight of the resin after 15 days at roomtemperature was 1797.

Comparative Example 4

Coating of Boron-Doped Silsesquioxane Solution

The boron doped silsesquioxane solution of Example 3 was coated onto a6″ blanket wafer and onto a pattern wafer using the spin, bake, and curecycles of Example 2. Film properties were listed in Table 2. Post-bakefilm thickness of 1100 Å on the blanket wafer was obtained from twocoating steps with a bake cycle between the coating steps. A goodquality film was obtained on a blanket wafer. The FTIR spectrum of thecured film showed that the silane (SiH) evidenced in the post-bake filmby peaks at 2000-2250 cm⁻¹ and 800-900 cm⁻¹ had been converted to silicaand as a result, a boron-doped silica film had been produced.

TABLE 2 Film Thickness and Shrinkage of Boron-Doped Silsesquioxane FilmFilm Thickness Refractive Index Shrinkage from Bake Post-bake 1110 Å1.410 — Post-cure  868 Å 1.438 21.8% Post-anneal  800 Å —   28%

The gap filling behavior of the material was observed from SEM picturestaken after each processing step. The boron-doped silsesquioxane filledgaps in the patterned wafer without void or delamination down to thenarrowest size of about 50-60 nm after the post-bake. After a 700° C.cure in nitrogen/oxygen, the silica formed showed no delamination, but alower density material was identified around the lower corners of thegap, based on SEM brightness of that region. After a 900° C. anneal innitrogen, delamination and cracking occurred due to further shrinkageinduced in the annealing.

EXAMPLE 5

Infiltration of Colloidal Silica Film with Boron-Doped Silsesquioxane

The 1.8 wt % colloidal silica solution from Example 1 was used to coat apatterned wafer, following the spin and bake conditions of Example 2.After bake, the boron-doped silsesquioxane solution from Example 3 wasspin coated onto the coated patterned wafer using the spin/bakesequences of Example 4. The infiltrated wafer was then cured andannealed using the cure and anneal conditions of Example 2.

The extent of gap-filling in the patterned wafer was determined byexamining the SEM pictures of the gaps of various sizes. For all thepost-bake, post-cure and post-anneal samples, there were no delaminationinside the gap of all sizes down to the narrowest size of about 50-60nm. An overlayer of boron-doped silica was formed on top of theinfiltrated colloidal silica film. The thickness of the overlayer wasabout 200 nm.

EXAMPLE 6

Etching Resistance

Silica coated blanket wafers of Example 2 boron-doped silsesquioxanecoated wafers of Example 4 were dipped into 500:1 BOE (buffered oxideetchant) solution containing ammonium fluoride for 180 seconds at anetch temperature of 21° C. After etching, the treated wafers were rinsedwith deionized water and the film thickness was measured. The etch rateof silicon oxide produced by CVD deposition of TEOS was used as areference for comparison. Etch rate was calculated as the decrease offilm thickness divided by the etch time. Etch rate in Å/sec and relativeetch rate with respect to that of CVD TEOS were listed in Table 3. Theaverage etch rate of the CVD TEOS was about 0.45 Å/sec.

TABLE 3 Etch Rate of Colloidal Silica and Boron-Doped SilsesquioxaneFilms Etch Rate Etch Rate relative to (Å/sec) CVD oxide Post-cureColloidal Silica 7.0 15 Post-anneal Colloidal Silica 1.7 3.8 Post-cureB-silsesquioxane 0.66 1.5

Coated patterned wafers of Examples 2, 4, and 5 were dipped in 500:1 BOEsolution for 20 seconds, followed by a deionized water rinse. SEMpictures of the etched samples were taken to examine the etch resistanceof the dielectric filled in the narrow gaps. Results are summarizedbelow.

TABLE 4 Etching of Patterned Wafers Heat Treatment Etching of GapDielectric Example 2 Post-cure no crack, no etching Example 2Post-anneal no crack, no etching Example 4 Post-cure severely etchedExample 4 Post-anneal crack before etching, etched out Example 5Post-cure no crack, no etching Example 5 Post-anneal no crack, noetching

The inclusion of silica nanoparticle in the dielectric has shown twomajor benefits: (1) elimination of cracking or delamination induced by700-900° C. heat treatment and (2) significant improvement in the etchresistance of the gap dielectric to BOE etching. These benefits are dueto the low shrinkage and high density achieved by using the inert fillercomposite approach.

EXAMPLE 7

Synthesis of Doped Silicates

To 10.0 gm of a 5% solution of P₂O₅ in 2-propanol, 0.38 gm B₂O₃ wasdissolved with stirring for 1 hour. 9 gm TEOS in 9.5 gm acetone wasadded followed by the addition of 1.38 gm 1N HNO₃ and 0.5 gm water. Themixture was heated to boiling around 66° C. and maintained for 3 hours.The molecular weight was about 2000 amu. The silicate resin wasestimated to have 8% by weight phosphorus and 4% by weight boron. Thesolid content of the solution was 8 wt %.

EXAMPLE 8

Infiltration of Colloidal Silica Film with Doped Silicate

A colloidal silica film was deposited onto a blanket wafer using the 1.8wt % colloidal silica solution from Example 1. The spin recipe was 5seconds at 300 rpm for dynamic dispense of 2 ml of solution followed bya 20 second spin at 2000 rpm. The wafer was baked at 80° C., 150° C. and250° C. for one minute each. The film thickness was 520 Å with arefractive index of 1.23.

The porous silica film was infiltrated with the doped silicate solutionfrom Example 7. A rinsing solution consisting of 50 wt % 2-propanol and50 wt % acetone was prepared. The following infiltrating and rinsingprocedure was used: (1) dynamic dispense of 4 ml of doped silicatesolution onto the wafer at 300 rpm for 5 seconds; (2) spread of liquidat 1000 rpm for 1 second; (3) rest for 5 seconds; (4) dynamic rinseusing 4 ml of rinsing solvent for 5 seconds at 300 rpm; and (5) finalspin at 2000 rpm for 20 seconds.

The infiltrated film was baked at 80° C., 150° C. and 250° C. for oneminute each. Film thickness after bake was 517 Å with refractive indexof 1.43. The increase of refractive index to the value of 1.43 showedthat a solid non-porous film was been produced.

EXAMPLE 9

Infiltration of Colloidal Silica Film with Doped Silicate

The 1.8 wt % colloidal silica solution of Example 1 was used to coat apatterned wafer using a dynamic dispense of 3 ml in 3 seconds at 300rpm, followed by a final spin of 2000 rpm for 20 seconds. One minuteafter spin without baking, 3 ml of doped silicate solution from Example7 was dispensed onto the wafer in 3 seconds while spinning at 300 rpm,followed by final spread at 2000 rpm for 20 seconds. The infiltratedfilm was then baked, cured and annealed according to the recipedescribed in Example 2. SEM images of the annealed film showed nocracking of the film in the gaps and in the top layer of the infiltratematerial. The infiltrated wafer was soaked in BOE 500:1 solution for 20seconds. No etching was observed around the corners of the wide andnarrow gaps in SEM pictures at 80000 magnification.

EXAMPLE 10

Premixing of Colloidal Silica and Doped Silicate Solutions

A 4 wt % solution of colloidal silica was prepared by adding 23 grams ofcyclohexanone to 15 grams of 10% colloidal silica stock solution. 9grams of TEOS was mixed with 9.5 grams of acetone, followed by theaddition of 1.38 g 1N nitric acid and 0.58 grams of deionized water. 10grams of 5 wt % P₂O₅ solution in isopropyl alcohol (IPA) was added andmixed well. The mixture was stirred for 72 hours at room temperature.The solution was then diluted with 15.23 gm of acetone and 15.23 gm ofIPA to give a final solid content of 4 weight percent P-doped silicate.

The following three solutions were prepared and spin-coated onto blanketwafers, followed by bake, cure and anneal as Example 2:

Solution 10A: 10 grams of 4% silica and 10 grams of 4% P-doped silicatesolution

Solution 10B: 16 grams of 4% silica and 4 grams of 4% P-doped silicatesolution

Solution 10C: 18 grains of 4% silica and 2 grams of 4% P-doped silicatesolution Results are shown in Table 5 below.

TABLE 5 Premixing of Colloidal Silica and P-Doped Silicate SolutionsShrinkage Etch Rate Solu- Process SiO₂: Film Refractive from in 500:1tion (° C.) Silicate Thickness Index bake BOE 10A 700 1:1 798 Å 1.4408.5% 1.46 Å/sec 10A 900 1:1 705 Å 1.441 19.2% 0.58 Å/sec 10B 700 4:1 750Å 1.380 5.6% 0.88 Å/sec 10B 900 4:1 699 Å 1.388 12.2% 0.61 Å/sec 10C 7009:1 805 Å 1.270 3.3% — 10C 900 9:1 776 Å 1.280 6.8% —

Although the present invention has been described in terms of specificmaterials and conditions, the description is only an example of theinvention's application. Various adaptations and modifications of theprocesses disclosed are contemplated within the scope of the inventionas defined by the following claims.

We claim:
 1. A process of forming a dielectric layer on a substrate, theprocess comprising: depositing a colloidal dispersion on a substrate,the colloidal dispersion comprising particles of a dense materialdispersed in a solvent, wherein the dense material is a dielectricmaterial or a material convertible to a dielectric material by oxidationor nitridation; drying the colloidal dispersion to form an intermediatelayer; infiltrating the intermediate layer with a matrix material toform an infiltrated layer; and curing the infiltrated layer whereby theinfiltrated layer is modified forming the dielectric layer.
 2. Theprocess of claim 1 wherein infiltrating the intermediate layer with amatrix material is infiltrating the intermediate layer with a coatingsolution of a spin-on polymer material.
 3. The process of claim 2wherein the spin-on polymer material comprises a material selected fromthe group consisting of silicates, hydrogen silsesquioxanes,organosilsesquioxanes, organosiloxanes, silsesquioxane-silicatecopolymers, silazane-based materials, polycarbosilanes, andacetoxysilanes.
 4. The process of claim 2 wherein the spin-on polymermaterial comprises a species selected from the group consisting ofarsenic, antimony, phosphorous, and boron.
 5. The process of claim 1wherein the particles of the dense material are particles comprising amaterial selected from the group consisting of silica, silicon, siliconnitride, silicon oxynitride, aluminum, aluminum nitride, and aluminumoxide.
 6. The process of claim 5 wherein the dense material furthercomprises a species selected from the group consisting of arsenic,antimony, phosphorous, and boron.
 7. The process of claim 5 wherein theparticles of the dense material comprise silica.
 8. The process of claim5 wherein the particles of the dense material comprise silicon.
 9. Theprocess of claim 1 wherein the particles have a characteristic dimensionbetween about 2 nanometers and about 50 nanometers.
 10. The process ofclaim 1 wherein the dielectric layer is a pre-metal dielectric layer onan integrated circuit device.
 11. The process of claim 1 wherein thedielectric layer fills a trench in a shallow trench isolation structure.12. The process of claim 1 wherein the dielectric layer fills gaps ofdimension less than 100 nanometers.
 13. The process of claim 12 whereinthe dielectric layer is heated above 700° C. without cracking.
 14. Theprocess of claim 1 wherein the dielectric layer is resistant to standardbuffered oxide etchant solutions.
 15. The process of claim 1 whereininfiltrating the intermediate layer with a matrix material is depositinga matrix material on the intermediate layer by chemical vapordeposition.
 16. The process of claim 1 wherein infiltrating theintermediate layer with a matrix material is depositing a matrixmaterial on the intermediate layer by atomic layer deposition.
 17. Theprocess of claim 15 wherein the matrix material is selected from thegroup consisting of phosphosilicate glass, borosilicate glass, andborophosphosilicate glass.
 18. The process of claim 1 wherein curing theinfiltrated layer whereby the infiltrated layer is modified is curingthe infiltrated layer whereby open porosity of the infiltrated layer isminimized.
 19. A process of forming a dielectric layer on a substrate,the process comprising: depositing a colloidal dispersion on asubstrate, the colloidal dispersion comprising particles of a densematerial dispersed in a solvent, wherein the dense material is adielectric material or a material convertible to a dielectric materialby oxidation or nitridation; drying the colloidal dispersion to form anintermediate layer; curing the intermediate layer at a temperature of atleast 600° C., whereby the intermediate layer is modified everywhere onthe substrate, forming the dielectric layer.
 20. The process of claim 19wherein the particles of the dense material are particles comprising amaterial selected from the group consisting of silica, silicon, siliconnitride, silicon oxynitride, aluminum, aluminum nitride, and aluminumoxide.
 21. The process of claim 19 wherein the dense material furthercomprises a species selected from the group consisting of arsenic,antimony, phosphorous, and boron.
 22. The process of claim 19 whereinthe particles of the dense material comprise silica.
 23. The process ofclaim 19 wherein the particles have a characteristic dimension betweenabout 2 nanometers and about 50 nanometers.
 24. The process of claim 19wherein the dielectric layer is a pre-metal dielectric layer on anintegrated circuit device.
 25. The process of claim 19 wherein thedielectric layer fills a trench in a shallow trench isolation structure.26. The process of claim 19 wherein the dielectric layer furthercomprises a spin-on polymer.